Limited Reservation

As a suggestion on how to reserve chains stopping after N steps, or at campaign A using 

/restapi/chained_requests/flow/reserve/<N>
/restapi/chained_requests/flow/reserve/<Campaign>

One could wire a pop-up to the "reserve chain" buttons to suggest to put the extra parameter

Extra work (just ask) would be required to pass this on at the *mccm* level (same comment on the pop-up)

 To be tested prior to merging.